In this fourth part in our series on computers, I begin with an overview of the von Neumann architecture used in most modern computers. This leads in a discussion of the structure and operation of the central processing unit, covering the instruction register, the program counter, main memory, the data and address buses, the ALU, and the control Unit. I then discuss the purpose and implementation of a number of key operations in the instruction set architecture, including arithmetic operations, data movement operations, and conditional branches.